Patented Feb. 10, 1931 1,792,410

UNITED STATES PATENT OFFICE MAx BUCHNER, OF HANOVER-KLEEFELD, GERMANY PROCESS OF PRODUCING ALUMINA FROM ALUMINIFEROUS SUBSTANCES No Drawing. Application filed March 7, 1928, Serial No. 259,906, and in Germany December 24, 1926. I have filed applications for patents in composition of aluminium nitrate can be ef Germany, December 24th, 1926; Germany, fected by heating the solution of aluminium January 3rd, 1927; Germany, February 17th, nitrate obtained or by heating the solid alu 1927; Germany, February 25th, 1927; Ger minium nitrate produced from the solution. 5 many, August 24th, 1927; Germany, Septem The oxides of nitrogen evolved are used, if 55 ber 5th, 1927; Germany, October 24th, 1927; desired, for reforming nitric acid for treat Great Britain, December 23rd, 1927; Great ing a fresh quantity of raw material. Britain, January 3rd, 1928; Great Britain, Examples of suitable acid-proof alloys are January 13th, 1928; France, December 24th, alloys of iron, chromium and nickel, alloys 10 1927; Italy, January 11th, 1928; Yugoslavia, containing tungsten and the like, and refined 60 November 9th, 1927; Yugoslavia, December steels...... w w 24th, 1927; Norway, December 23rd, 1927; For decomposing the aluminiferous ma Austria, December 23rd, 1927; Switzerland, terial it has been found advantageous to treat December 24th, 1927; Czechoslovakia, De the damp or air-dry raw material with di 15 cenber 31st, 1927; Czechoslovakia, January lute or concentrated acids. In this way it 3rd, 1928; Czechoslovakia, January 9th, 1928; is possible to minimize the difficulties at Hungary, December 24th, 1927; Hungary, tending the production of pure alumina, and January 3rd, 1928; Hungary, January 14th, in particular alumina free from iron. Thus, 1928. - for example, in the case of a bauxite con 20 Attempts have been made to prepare alu taining 50% of iron oxide, 98% of the iron 70 minium oxide by thermal decomposition of present can be dissolved out by simple pre aluminium salts, such as aluminium chloride, treatment with concentrated hydrochloric aluminium sulphate and ammonia alum. The acid. The content of aluminium is practi last mentioned process was found to be eco cally unaffected. - . 25 nomically impracticable. Aluminium sul In the same manner the raw material can 75 phate must be decomposed at such a high be treated with nitric acid. The production temperature that the greater portion of the of the aluminium nitrate solution, that is the sulphuric acid is decomposed into Sulphur extraction of the alumina from the raw ma dioxide and oxygen, so that it is only possible terial, is preferably carried out by heating 30 to recover the sulphuric acid by a complicated the raw material with nitric acid. - 80 method, or by means of expensive apparatus. According to the nature of the material The decomposition of hydrated aluminium to be extracted it may be necessary to pro chloride is attended with great technical dif long the heat treatment, in order to obtain ficulties and cannot be carried out in a sim a larger yield of alumina. 35 ple, direct manner, owing to the destructive A particular advantage is the preparing action of the hydrochloric acid. of the solution of aluminium nitrate by have found that nitric acid enables alu treating an excess of aluminiferous material minium oxide to be produced from clay and related as to the content of alumina with other aluminiferous substances in a very sim nitric acid. In spite of the deficiency of ple and economical manner. nitric acid it is surprising how much more 90 According to my invention alumina is pro clay passes into solution than would be as duced from clay and other aluminiferous sub sumed for the composition of normal alu stances by treating the raw material, which minium nitrate. When working, for example, may be previously calcined, for example at at a temperature of 80 to 100° C. about 30 45 temperatures from 550° to 800° C., with nitric to 40% more aluminium oxide goes into so- 95 acid in vessels made of acid-proof alloys, the lution than corresponds to the equivalent resulting solution of aluminium nitrate be quantity of nitric acid. ing separated from the insoluble residue, and The particular advantage of producing the alumina being obtained from the said so basic aluminium nitrate is that, when using 50 lution by thermal decomposition. The de aluminiferous materials containing iron, the 100 1,792,410 elimination of iron occurring simultaneously The separation of impurities can be ef is so thorough, that the aluminium oxide pre fected by other means. If a solution of pared from basic aluminium nitrate con aluminium nitrate is prepared in the first tains only about 1 to 1.5% of Fe2O3. Prob instance and is subjected to the action of ably this is due to the fact that aluminium pressure, after removal of the insoluble resi to oxide is dissolved in excess. due and if desired after being concentrated, If the solution of basic aluminium nitrate, the impurities are removed, prepared with a deficiency of nitric acid, is It is preferable to carry out this pressure allowed to stand for a few hours, or a longer treatment in stages; thus, for example, the 10. period, or if it is previously cooled, a fur mixture of aluminiferous raw material and ther reduction of the iron content...of the nitric acid, or the aluminium nitrate solu s alumina is effected to about 0.7% Fe.O.s. In tion freed from insoluble residues, may be some circumstances the cooling may be con heated to between 80° and 100° C. the tem tinued until the solution solidifies. I have peratures and pressure being then gradually 5 then found that after crystallizing out the increased stepwise. : ------basic aluminium nitrate the elimination of The advantage of heating and raising the iron has been so thorough, that 100 grams of pressure in stages is that the apparatus is not aluminium oxide prepared from it contains Subjected for long to the corrosive action of no more than 0.1 to 0.05% of FeOs. The thereagents, as would otherwise be the case. same result is obtained if the solution of A further method of removing iron and basic aluminium nitrate is evaporated to the other impurities is to treat the solution of point of crystallization. The basic alumin aluminium nitrate with a concentrated solu ium nitrate is also freed from other impuri tion of nitric acid or gaseous nitric acid. ties by a single crystallizing operation. As treating with nitrate acid in this way the alu compared with results obtained with other minium salt is precipitated. For example, aluminium salts, particularly with reference by adding double the quantity of concentrated to the elimination of iron, basic aluminium nitric acid to a cold saturated solution of alu nitrate appears to occupy a unique position. minium nitrate, more than 80% of the all The period of treatment may be consider minium nitrate in solution may be precipi 3. ably reduced, with saving of heat, if the tated in the form of fine crystals. Even bet: aluminiferous raw material and the nitric ter results have been obtained by using gase-I - acid are first made into a paste, and if the ous nitric acid. After treating the alumin- - paste is stirred and allowed to stand for a ium nitrate solution in this way the impuri long time at lower temperature, to form a ties remain in solution and can be easily re kind of sludge. Then the aluminium nitrate moved. . is produced by heat with the addition of A high degree of purification, particularly further quantities of nitric acid. with regard to iron, is thus obtained during Thus if the extraction process is applied the decomposition or treatment of the alumia to calcined clay, with heat and stirring, but ium nitrate solution, and the last traces can without the pre-treatment with nitric in the be readily and completely removed in the as cold, a yield of 78% of alumina may be ob course of the process if the aluminium nitrate tained, but if pre-treated in the manner de Solution, after being concentrated, is treated scribed a yield of 98.7% may result. with potassium ferro-cyanide in concen Impurities, for example, iron, titanium, trated solution or in the solid state. If the 45. etc., may be separated in the course of the solutions are very highly concentrated a pre O process. For example, a high degree of cipitate of the ferro-cyanogen compound is purification may occur when the aluminif obtained, which can be easily filtered off. erous raw material is treated by nitric acid, For efficient precipitation of the iron it is ad with heat and under pressure, with or with Vantageous to apply heat, stir the mixture out prior treatment of the raw material to and keep the reaction neutral. form a sludge. Thus iron and other impuri The ferro-cyanogen compound obtained ties may be separated in a surprisingly effec can be used as a blue pigment, or it may be tive manner, furthermore the decomposition used for re-forming the original cyanogen takes place more rapidly and there is a more compound, for example, by treatment with complete solution of the alumina. alkali, ammonia or alkali carbonate. In this Thus, I have found when bauxite was Way ferric hydroxide and a cyanogen com treated for six hours with nitric acid at 8 pound are obtained, the latter being used atmospheres pressure, and 162° C. about again in the process. - 95% of its content of aluminium oxide was Another method of removing the last traces dissolved. Working without the application of iron from the aluminium nitrate solution iss of pressure, for the same period and at a is to treat it with a thiocyanate. By this temperature of 95° C. (a temperature which treatment iron thiocyanate is formed which cannot in practice be exceeded without the can be removed by means of ether. For re application of pressure) only about 75% of covering the residue the ether is evaporated, ; the aluminium oxide passed into solution. and the residual iron thiocyanate is treated 80 1,79 2,410 3. with an (alcoholic) solution of alkali, ann the solution can be crystallized out at a tem monia or alkaline carbonate for recovering perature above 47.5°C., and in this way it is the thiocyanate. . . obtained in a purer form. . Even though cyanogen compounds, such as The crystals of aluminium nitrate so ob thiocyanogen compounds, have a relatively tained are generally washed with nitric acid. 70 high market value, it is possible to use them It is preferable to wash with concentrated economically in the process as there is no nitric acid, as in this way impurities are re special difficulty in recovering them practi moved. Purification is also effected by treat cally without loss...... ing solid aluminium nitrate or a paste of crys O The solution of aluminium nitrate ob tals thereof with gaseous nitric acid. 75 tained by the extraction of the aluminiferous The decomposition of aluminium nitrate raw material, and partly or completely freed may be effected by heating it in the solid, from iron, is used for the production of crystallized, or dissolved state, or in the state alumina. The reforming of the nitric acid in which it occurs after the last step in the 5 from the alminium nitrate is important and process, if desired, with the introduction of 80 advantageous. In order to carry out this re gases or of water or steam. In this way nitric covery with a minimum of loss, the solution acid of any desired concentration can be pro of aluminium nitrate is first highly concein duced. trated by evaporation in vacuo, till the alu it is preferable to carry out the evapora 20 minium nitrate crystals are fused in their tion of the aluminium nitrate solution in the 85 water of crystallization. This “melt is then presence of air, oxygen and steam, and also more strongly heated so that the oxides of neutral gases (nitrogen, carbon dioxide), and nitrogen are evolved in a highly concen it is equally advantageous to effect the de trated condition. It is sufficient to work at composition of the aluminium nitrate with 25 relatively low temperatures, so that it is the introduction of gases, whether the heat possible to use the ordinary apparatus with ing and decomposition are effected in one op external application of heat. A rotary kill eration or in two steps, and whether the ac may be used. tion is carried out in vacuo, in partial vacuo The indicated temperature conditions are or under pressure. 30 particularly advantageous in practice for the it has been found preferable to effect the decomposition of aluminium nitrate. For decomposition of the aluminium nitrate in example, temperatures between 300 and vacuo. The decomposition into aluminium 500° C. are quite suitable, but with the opei'- oxide and nitric acid then takes place easily ation carried out in two stages, i. e. the re and steadily at relatively low-temperatures, 35 moval of water and the decomposition, high and nitric acid of very high concentration is CO er or lower temperatures may be used. readily produced. Instead of effecting the removal of basic in this way about 92% of the nitric acid impurities during the decomposition or ex in the aluminium nitrate can be recovered, traction, or in addition to doing this, solid therefore nitrous gases are formed only in 40 aluminium nitrate or a paste of crystals Small quantity. Even at 145° C., 75% of the thereof may be produced by evaporation or nitric acid may be distilled over, and the cooling. This method is particularly ada civain remainder at about 2008. These remarkably tageous in the case of a solution of basic all good results are obtainable at an absolute minium nitrate, produced by effecting the pressure of 220mms. 5 decomposition with a deficiency of nitric The preparation of alumina from alumin acid. The removal of iron is particularly ium nitrate can be carried out by decomposi thorough if precautions are taken to bring the tion in vacuo, equally well whether the alu iron present in the reacting solution into the minium nitrate is in crystalline or dissolved colloidal form. For example, this may be ef condition, or if free nitric acid is present O fected by adding bases, such as aluminina during the decomposition. The degree of oxide, alkaline earths, or alkalies, to the solu concentration of the nitric acid obtained in tion of decomposition products obtained, un addition to aluminium oxide is of course de til the required degree of basicity is obtained pendent on the water present, or the water Alternatively the solution may be evaporated formed by decomposition. If aluminium ni 55 until sufficient nitric acid vapour has been trate containing very little water is to be driven off to produce the required degree of decomposed, and if it is desired to produce basicity. nitric acid of a given concentration it is nec Another method of removing inpurities essary to add the requisite quantity of Water, to evaporate the solution of aluminium or to introduce steam during the distillation. 60 trate and then to add concentrated or gaseous The decomposition of aluminium nitrate nitric acid. I have already indicated that, by in vacuo can be effected also in presence of adding concentrated nitric acid aluminin free nitric acid. nitrate is precipitated. If the aluminium ni Furthermore it is very favourable to the trate solution is simultaneously evaporated, decomposition of the aluminium nitrate to 65 the greater part of the aluminium nitrate in have alumina present. The decomposition

4. 1,792,410 proceeds in such manner that as the nitric nitrate may be evaporated down with alumia. acid is distilled off aluminium nitrates of ium nitrate, to be decomposed and used as a

increasing basicity are formed. The basic melt for effecting decomposition. A mixture aluminium nitrate swells up when heated, of sodium and potassium nitrates may be forming films and viscous, greasy bodies, used instead of the sodium nitrate. Calcium - which resist the conduction of heat. By add nitrate, in particular hydrated calcium ing alumina to the decomposition nitrate this nitrate, may be used as a melt. is avoided. For example, a substance con When the decomposition is carried out with taining a high percentage of alumina may the aid of a melt it is convenient to work in i. be added to the nitrate in the proportion of vacuo. It is also preferable to introduce one part to two parts. The alumina may, gases, for example, steam, for driving out the however, be added to the evaporated Solution nitric acid. . of aluminium nitrate, and the mixture de As already mentioned in connection with composed. Another method is the spraying the decomposition of aluminium nitrate in a 5. of the evaporated solution of aluminium ni melt, the introduction of gases during the trate mixed with alumina. . . . . heating operation is advantageous. A Another method of decomposing the alu The aluminium oxide obtained by the de minium nitrate is to introduce the material coun)osition may be further purified. It has to be decomposed into a suitable “melt” or been shown that any remaining iron com 20 into a liquid of suitable boiling point, if nec pounds or other impurities can be practically essary with agitation, and to distill off the wholly removed by treatment with concen oxides of nitrogen at the required decompo trated nitric acid. Alternatively the alumin sition temperature and at atmospheric pres ium oxide obtained by decomposing alumin sure, or under partial vacuum, with the in ium nitrate may be dissolved in soda lye, if troduction of steam, air, oxygen or gas. necessary with the application of heat in an The advantages of the introduction of autoclave. The sodium aluminate obtained is steam or gases, are that the partial pressure filtered and the aluminium separated by pre can be reduced and that more complete con cipitation in known manner...... version of the decomposing gases into nitric For the extraction of the raw material, for S. acid is obtained. By carrying out the de the production of the solutions of aluminium composition in a melt or liquid at the required nitrate, dilute nitric acids can be used.

temperature better heat transference is ob Therefore nitric acid used for washing the precipitated aluminium nitrate or the all. tained. Suitable materials for the melt are molten salts which are stable at the required mina, generally for purifying in course of the 5 temperature, or molten metals. Organic liq process, can be used for extracting or with e uids are also suitable. The nitrate to be de the heat treatment of the raw material. By composed is also incorporated in the melt. evaporating the solution and by decomposi Under the conditions mentioned the decom tion of the aluminium nitrate, highly concen, position in the melt proceeds uniformly and trated or gaseous nitric acid can be reformed completely, and much more uniformly than in a very simple manner and reused in the with other methods of decomposition. It is p rocess. probable that the melt at least in some cases All steps of the process can take place also acts as a catalyst. in vessels made of acid-proof alloys, thus the It is a great advantage that practically no extraction or dissolving, the heat treatment, nitrous gases are formed, as this would give the decomposing-reaction etc. - rise to a loss of nitric acid. . . . . Examples of suitable acid-proof alloys, al For example, sodium nitrate may be used ready mentioned above, are alloys of iron, as a melt. As this salt melts at slightly above chromium and nickel, alloys containing tung 300° C. it forms a suitable melt (heating sten and the like, and refined steels. It is re bath), and enables the decomposition of the markable that vessels made of such materials aluminium nitrate to be rapidly and permit of treatment under pressure in spite thoroughly effected in simple apparatus, such of the presence of nitric acid. as a boiler having stirring means. When The process enables very pure alumina to steam is introduced, or water added, nitric. be obtained in a very simple manner, and no acid and aluminium oxide result, the latter in reagent used need be lost in the form of a heavy, granular form, which after completion valuable but unrecoverable by-product; both of the reaction and upon treating the melt the extracting and purifying agents can be with water, sinks to the bottom of the aqueous recovered in the course of the process, and are solution. Thus the separation of the all repeatedly used in the process. This applies i: iminium oxide in the melt is simplified, the particularly to the nitric acid used for ex upper portion of the melt being first decanted tracting or for the heat treatment of the raw from the oxide. The separation of the all material, and to nitric acid used in the con minium oxide from the sodium nitrate is ef centrated or gaseous form for removing iron fected by filtration after extraction with from the aluminium nitrate solution, or from water. The resulting solution of sodium the solid aluminium nitrate, or from the crys l

1,792,410 5 talline paste thereof, or for washing the pre containing about 102 kg. of Al-O, contains cipitated aluminium nitrate. iron to the extent of about 2% Fe.Oa in rela tion to the quantity of AlOs in solution. Eaamples ... : This solution after being concentrated is 1. 100 kg. of clay calcined at 500° C. are subjected to pressure as set forth in Example 70 treated with 200 litres of nitric acid of 400 3. A brown precipitate is formed. There is grams per litre strength for four hours at practically no loss of aluminium nitrate from about 80° C., the mixture being stirred. The the Solution...... hot solution is then freed from insoluble resi 'he loss amounts to about 1 to 2% of dues by means of a suction filter. The filtrate AiO3. The quantity of iron in the solution 75 contains about 140 kg. of Al (NO3)39H2O in drops to about 0.6% of Fe2O3 with reference: solution, about 2.2 kg. Fe(NO3)39H2O and to the amount of Al-Oa in the solution. Small quantities of alkaline earths and al claim: . . . kalies. All the silicic acid and the titanic 1. The method of producing pure alumina, acid together with the greater part of the iron which comprises treating aluminiferous ma 80 5 remain in the residue. The solution is then terial with nitric acid insufficient to combine evaporated in vacuo and subsequently heated with all the alumina in the material, sepa to 500°. until the nitrate is completely decom rating the aluminum nitrate solution and dis posed. The oxides of nitrogen evolved are Sociating the latter by heat...... collected and are used for a subsequent ex 2. in the method of producing pure 85 20 traction. There remains 19 kg. of Al2O3. alumina, treating aluminiferous material for The resulting alumina, which contains about several hours with nitric acid insufficient to 0.45 kg. of FeO3, is dissolved in soda lye with combine with all the alumina in the material or without the application of pressure, and and at a temperature of 80°-100° C. 90 5 is further treated in known manner, for in 3. In the method of producing pure stance according to Bayer's method. The loss alumina, mixing aluminiferous material to of alkali is thus considerably less than by a paste with nitric acid, allowing the mixture direct alkaline decomposition of bauxite or to stand at a temperature above atmospheric clay. temperature, then adding additional nitric 95 30 2. 100 kg. of crystals of aluminium nitrate, acid, the total acid being insufficient to com which are cbtained by concentrating the ni bine with all the alumina in the material. trate solution, preferably a solution contain 4. The method of producing alumina, ing nitric acid as it occurs after the reaction, which comprises treating aluminiferous ma and which need not be separated from the terial under heat and pressure with nitric 5 mother liquor and which contains about 2.2% acid insufficient to combine with all the 100 FeOs with reference to the AlOs), are alumina present, and decomposing the result- : twice washed in the cold with concentrated ing aluminum nitrate into alumina and acid. nitric acid (sp. gr. = 1.4). After this treat 5. In the method of producing pure ment the iron content falls to about 0.5% alumina, mixing aluminiferous material and 4 a Fe.O., while only traces of aluminium nitrate nitric acid insufficient to combine with all the 05 pass into solution. The resulting nitric acid alumina of the material while subjecting it containing Small quantities of iron can be stepwise to increase of heat and pressure, and used again for the purifying process or for decomposing the resulting aluminum nitrate extraction. by heat. 4 5 3. 490 kg. of calcined clay, having an 6. In the process of producing pure alu 10 alumina content of about 23% are treated mina, treating aluminiferous material with with 1000 litres of nitric acid (400gm. HNO. nitric acid insufficient to combine with all the per litre) for about four hours at 8 atmos alumina of the material, separating the solu pheres pressure and at a temperature of about tion of aluminum nitrate and decomposing 5 0 160° C. The aluminium nitrate solution pro it under a pressure less than atmospheric 5 duced (containing about 100 kg. of Al2O3) is pressure while heating it. practically pure, the iron content in relation 7. In the process of producing pure alu to the content of Al2O3 amounting to only mina, treating aluminiferous material with 0.7%. nitric acid insufficient to combine with all the 5 5 After separating the residue, the nitric 20 acid used is recovered for a subsequent ex alumina of the material, separating the solu traction during the thermal decomposition tion of aluminum nitrate and decomposing of the aluminium nitrate. it under a pressure less than atmospheric 4. The solution obtained by treating 525 pressure while heating it in an atmosphere of 60 kg. of calcined clay (containing about 23% a gas indifferent to nitric acid. 125 A.O.) with 1000 litres of nitric acid (con 8. In the process of producing pure alu- ..., taining 400gm. HNO per litre) at the ordi mina, treating aluminiferous material with nary pressure and at a temperature of about nitric acid insufficient to combine with all the 80° C. is separated from the undissolved alumina of the material, separating the solu 5 residue. The solution of aluminium nitrate tion of aluminum nitrate and decomposing 130

6 1,762, dio it under a pressure less than atmospheric pressure while heating it in nitric acid. 9. In the process of producing pure alu mina, treating aluminiferous material with nitric acid insufficient to combine with all the alumina of the material, separating the solu tion of aluminum nitrate and decomposing it under a pressure less than atmospheric pressure while heating it in nitric acid and gaseous nitric acid. 10. In the process of producing pure alu mina,nitric acidinsificienttreating aluminiferous to combine material with all with the alumina of the material, separating the solu tion of aluminum nitrate and decomposing pressure,it under anda S'E, treating less the than resulting atmospheric alumi hum oxide with concentrated nitric acid to remove11. The impurities. process of making pure alumina, which comprises treating aluminiferous ma terial in ferro-chrome-nickel alloy containers with nitric acid insufficient to combine with all the alumina of the material, separating the aluminum nitrate solution and decom posing the nitrate by heat to obtain alumina. In testimony that I claim the foregoing as my invention, I have signed my name. . OR. -MAX BUCHINER. 30 sis

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